To fulfill Santé publique France’s mandate to assess the impact of seasonal influenza, a “real-time” surveillance system—essential for tracking the progression of the epidemic—has been in place for several years. It enables the early identification of the key characteristics of the current epidemic. Two indicators of influenza severity are used in real time: the number and proportion of hospitalizations following visits to the emergency room due to influenza, and cases of severe influenza admitted to intensive care. The first indicator is provided by the Oscour® system and the second by the 192 sentinel intensive care units that transmit the information to Santé publique France. These two surveillance systems, which are independent of one another, do not allow for a complete estimate of the burden of seasonal influenza epidemics on hospitalizations.

Our study was conducted as part of the activities of the Professional Practice Exchange Group on Acute Respiratory Infections (Gepp IRA). The objective was to provide a more comprehensive assessment of the severity and impact of influenza on hospitalizations. The study focused on analyzing hospital stays from the Information Systems Medicalization Program (PMSI), which contains data on all hospitalizations in France.

Our study analyzed 91,255 hospitalizations with a diagnosis of influenza that occurred in mainland France during the five winter seasons between 2012 and 2017. Hospitalization rates and severity criteria (admission to intensive care, in-hospital mortality, length of stay) were analyzed by age group and by season.

Each seasonal influenza epidemic is characterized by the circulation of one or more dominant subtypes (B/Yamagata in 2012–13, B/Victoria in 2015–16, A(H3N2) in 2014–15 and 2016–17, A(H1N1) and A(H3N2) in 2013–14). Our results showed that hospitalization rates in each age group were highly dependent on the subtypes circulating each year. Thus, hospitalization rates were high among the elderly during years when the A(H3N2) virus was circulating, and high among young people during the year when the B/Victoria virus was circulating. The differing impacts observed by age can be explained by the characteristics of the virus subtypes, particularly the duration of their circulation and their recent antigenic evolution. It is therefore important to stratify analyses by age when assessing the impact of influenza epidemics.

Furthermore, our results highlight the importance of age on the severity criteria for hospitalizations. Over the study period, the proportion of deaths increased steadily with age, rising from 0.5% among those under 20 years old to 7% among those aged 60–79 and 10% among those aged 80 and older. The proportion of patients requiring intensive care was highest in the intermediate age groups of 40–79 years. Finally, age was a significant factor in prolonging the length of hospitalization, ranging from 2 days among those under 20 to 9 days among those aged 80 and older.

An analysis of hospitalizations with a diagnosis of influenza based on the PMSI provides insights into influenza surveillance data that are not available in other surveillance systems. The delay in obtaining data from this database (6 months) is incompatible with real-time analysis. However, these data could help supplement the epidemiological assessment during the annual review at the end of the flu season. Since PMSI data are available at the regional level, these analyses can also be applied at this scale and serve as a tool for regional health agencies to estimate the impact of influenza on hospitals.

These results help document the severity of influenza by age group and according to virus subtypes and can provide a benchmark for anticipating the impact of epidemics on hospitalizations. Regarding the influenza epidemic currently affecting mainland France, we are seeing the co-circulation of the A(H1N1)pdm09 and B/Victoria viruses. This current epidemic bears similarities to that of the 2015–2016 season. The impact of that epidemic on hospitalizations was moderate, with a low hospitalization rate among the elderly.

The scenario of a pandemic spread of the novel coronavirus (COVID-19) currently appears to be the most likely hypothesis. Preparing the healthcare system, particularly hospitals, will require developing scenarios for the spread of the virus and its impact on operations. These scenarios will be guided by an analysis of the situation in other countries that experienced the spread of COVID-19 before France.
In addition, seasonal flu epidemics serve as benchmarks for respiratory virus outbreaks, which French healthcare facilities have already experienced. The results of our study can serve as a reference for healthcare facilities in preparing their organizational response.